1. Field of the Invention
The present invention relates to a laminated polyester film for a magnetic recording medium. More particularly, the present invention relates to a laminated polyester film for a magnetic recording medium which achieves cost reduction, has improved slitting properties and generates no or few flaws or abraded powder during traveling when it is used as a base film of a magnetic tape.
2. Description of the Related Art
A polyester film has excellent mechanical and chemical properties and is widely used in various industrial fields. In particular, a biaxially stretched polyethylene terephthalate film is indispensable as a base film of a magnetic recording medium, since it has better flatness, mechanical strength and dimensional stability than other films.
Recently, properties of the magnetic recording medium has been very quickly improved. With the improvement of the magnetic recording medium, the base film for the magnetic recording medium is increasingly required to have better properties. For example, in the case of a high density recording medium such as a video tape, a surface of the base film should be extremely smooth.
However, as the smoothness of the base film surface is increased, some troubles arise. For instance, when a tape and a metal pin contact each other at a high relative speed as in the case of a magnetic tape, friction and abrasion between them increase so that scratches are formed on the film, or the film is abraded to generate powder.
To solve the above problems, it is proposed to compound particles having a large Moh's hardness such as aluminum oxide particles in the base film (see Japanese Patent Kokai Publication No. 306220/1989).
But, in the above technique, a large amount of the aluminum oxide particles should be compounded to prevent the scratches or the generation of powder sufficiently. As the result, the following new problems arise:
(1) Since the aluminum oxide particles are comparatively expensive, it increases the production cost of the magnetic recording medium disadvantageously.
(2) In the slitting step in the production of the film or the magnetic recording medium, a cutting blade is damaged rather quickly so that the cutting blade should be changed frequently. This leads to the increase of the production cost of the magnetic recording medium or the decrease of productivity.
In the slitting step in the production of the magnetic tapes, when the cutting blades are not changed at suitable intervals, a cutting property of the magnetic tape is deteriorated. The deterioration of the cutting property results in peeling-off of a cut area in a line form and, in some cases, drop-off as cuttings. When such phenomenon is worsened, powder drops off from a magnetic layer of the magnetic recording medium. The dropped powder will cause drop-out.
To improve the cutting property, it is known to increase a crystallinity of the film. Since increase of the crystallinity deteriorates wear resistance of the film, it is unpractical to increase the crystallinity to a sufficient level for the increase of the cutting property.
To solve the above described problems, it may be possible to laminate films and add particles having a large Moh's hardness only to the outermost layer so as to decrease a total amount of the particles in the laminate film. However, in the magnetic recording film, it is desired for the base film to have substantially the same physical properties on both surfaces like a monolayer film. If the above method is simply applied to the base film of the magnetic recording medium, the both surfaces of the film tend to have different surface roughness or different slip properties. A reason why the physical properties of the both surfaces of the film are required to be substantially the same is that, if they are different, a specific surface should be used for the formation of a magnetic layer.